Human sera are reactive with preparations of mammalian type-C viruses of simian, feline and murine, but not avian, origin. The reactant in serum has been localized to antibody combining sites of immunoglobulins, but the reactant(s) in virus preparations is not clearly identified. The reactions are inhibited by small quantities of viral gp70, but not p30, p14 or p12, and are also inhibited by cells producing pg70 but not p30 or p12. However, reactivity is not seen in direct radioimmunoassay with isolated I125 labeled gp70, and reactivity is seen with antisera to fetal bovine serum proteins. Further characterization of the reactant in viral preparations is under active investigation. BIBLIOGRAPHIC REFERENCES: H.W. Snyder, Jr., T. Pincus and E. Fleissner. Antigens in preparations of simian sarcoma virus/simian sarcoma-associated virus reacting with human sera in radioimmunoprecipitation assays. Comparative Leukemia Research 1975, Bibl. Haemat., No. 43, Ed. J. Clemmesen and D.S. Yohn, pp. 555-558 (Karger, Basel 1976). H.W. Snyder, Jr., T. Pincus, and E. Fleissner. Specificities of human immunoglobulins reactive with antigens in preparations of several mammalian type-C viruses. Virology 75:60-73, 1976.